Magnetar giant flares and afterglows as relativistic magnetized explosions

Abstract

We propose that giant flares on soft γ-ray repeaters produce relativistic, strongly magnetized, weakly baryon-loaded magnetic clouds, somewhat analogous to solar coronal mass ejection (CME) events. The flares are driven by unwinding of the internal non-potential magnetic field which leads to a slow build-up of magnetic energy outside of the neutron star. For large magnetospheric currents, corresponding to a large twist of the external magnetic field, the magnetosphere becomes dynamically unstable on the Alfven crossing time-scale of the inner magnetosphere, t A ∼ R N S/C ∼ 30 μs. The dynamic instability leads to the formation of dissipative current sheets through the development of a tearing mode. The released magnetic energy results in the formation of a strongly magnetized, pair-loaded, quasi-spherically expanding flux rope, topologically connected by the magnetic field to the neutron star during the prompt flare emission. The expansion reaches large Lorentz factors, r ∼ 10-20, at distances r ∼ 1-2 x 10 7 cm, where a leptophotonic load is lost. Beyond this radius plasma is strongly dominated by the magnetic field, though some baryon loading, with M <<E/c 2 , by ablated neutron star material may occur. Magnetic stresses of the tied flux rope lead to a late collimation of the expansion, on time-scales longer than the giant flare duration. Relativistic bulk motion of the expanding magnetic cloud, directed at an angle 0 ∼ 135° to the line of sight (away from the observer), results in a strongly non-spherical forward shock with observed non-relativistic apparent expansion and bulk motion velocities β app ∼ cot 0/2 ∼ 0.4 at times of the first radio observations, approximately one week after the burst. An interaction with a shell of wind-shocked interstellar medium (ISM) and then with the unshocked ISM leads to a deceleration, to non-relativistic velocities approximately one month after the flare.